Chromium plating process



United States Patent 3,418,221 CHROMIUM PLATING PROCESS Roger M. Woods, Washington, D.C., and David R. Moul, Seat Pleasant, Md., assignors to Corillium Corporation, Arlington, Va., a corporation of the District of Columbia No Drawing. Filed Mar. 16, 1966, Ser. No. 534,626

Claims. (Cl. 20451) This invention relates to a process of chromium plating and more particularly to a process for producing a lustrous uniform iridescent chromium finish on a Wide variety of conductive base materials.

An object of this invention is to provide a process of electro-depositing an extremely adherent corrosion resistant iridescent film upon a conductive base which film exhibits excellent wear resistance and which protects the base metal from corrosion. Another object of the invention is to provide a process using conventional chromium plating equipment wherein certain chemical compounds and conditions are provided for obtaining iridescent chromium plate. Another object of the invention is to provide a process for finishing decorative trim components for use in the automotive and appliance fields wherein iridescent chromium plate is obtained which has an attractive and unique decorative appearance and also has excellent wearing properties. Other objects and advantages of the invention will become readily apparent from the following detailed description of the invention.

According to the invention, an iridescent chromium plate is obtained on a conductive base material in a system in which the base metal is cathodic and in which the electrolyte comprises an aqueous solution of chromic anhydrides (CrO containing sulfuric and oxalic acid in the solution. The solution contains from about grams to 450 grams per liter of chromic anhydride and from about 0.07 gram per liter to about 2.1 grams per liter of sulfuric acid, and from about 1 gram per liter to about 75 grams per liter of oxalic acid. The sulfuric acid and the oxalic acid are preferred, although salts of these acids may also be used, such as sodium sulfate and ferric ammonium oxalate.

The object to be plated is prepared for plating by conventional plating procedures, which may include degreasing, alkaline cleaning, acid dipping, polishing, sandblasting, etc.

The preferred electro-plating bath comprises an aqueous solution containing to grams per liter of CrO and 0.16 to 0.26 gram per liter of sulfuric acid, and 10 to 20 grams per liter of oxalic acid.

The plating bath is prepared in the concentrations as stated above and added to a plating tank to the desired operating level. The tank must be thoroughly cleaned to prevent bath contamination, particularly where chloride or fluoride type baths have previously been used. Anodes are then placed in the tank and electrolyzed with dummy cathodes for about ten minutes. The solution is frequently stirred during this period as by air agitation. The bath is then ready for use. The object or part to be plated is then immersed in the bath and is made the cathode of the electro-plating system. Anodes of various materials may be employed, but it has been found preferable to use chemical lead or lead alloy anodes over the carbon or steel anodes. Steel and carbon anodes may be employed, but with less satisfactory results. The tank which holds the electrolyte solution is preferably lined with a synthetic resin material such as polyvinylchloride or other suitable inert material. The plating tank should also be provided with an exhaust system to carry away fumes of the chromic acid.

The plating bath should be operated at a temperature of about -140" F. and preferably at a temperature of 3,418,221 Patented Dec. 24, 1968 about -120 F. The best deposits will usually be obtained at a cathode current density of between 50 and 600 amperes per square foot, and preferably about 250 amperes per square foot. The current density is varied within the ranges given depending upon the complexity of the object to be plated.

Plating times will vary according to the thickness desired and the complexity of the object to be plated, but normally the time will vary between about 30 seconds to about ten minutes for decorative applications.

After the object has been plated, it is removed from the bath and rinsed in a conventional manner and dried by air blast. The covering power is slightly less than that of conventional chromium plating, and in cases of extremely deep recesses, internal anodes may have to be used, the same as in conventional chromium baths.

It has been found desirable to maintain the anode area as large as possible with respect to the cathode area in order to maintain the anode current density at a relatively low value to avoid coating the anode with a film, the exact nature of which is not fully understood, but which tends to increase the resistance of the system. The anode area should be as great as and preferably three or four times as great as the cathode area for most efiicient operation.

The procedure according to this invention produces color effects in the chromium plate which are decorative and pleasing to the eye. The iridescence produced may be varied from bluish to reddish hues depending upon the temperature used in the plating bath. At the lower temperatures, a bluish iridescent cast is obtained. At the higher temperatures a reddish iridescent cast is obtained. In the intermediate temperatures, a greenish iridescent cast is obtained.

The plating bath may be replenished by the addition of chromic anhydride and sulfuric acid and oxalic acid as they are consumed to bring the concentration of the ingredients in the bath back to the concentration of the ingredients originally present in the bath. A suflicient accurate control for production use may be achieved by conventional analytical techniques.

The plating bath of the invention which produces a uniform iridescent chromium finish is applicable to a wide variety of conductive base materails such as copper, nickel, steel, stainless steel, brass and bronze. Other metals such as zinc die cast and aluminum can also be plated after a suitable underplate is applied. For decorative deposits the thickness will be on the order of 5 to 30 millionths of an inch. For certain applications, thicker deposits may be applied by plating for a longer period of time.

The plate produced by the above'described process is extremely lustrous and iridescent in color and exhibits an extremely lustrous appearance when plated over a bright or polished surface. Spectral reflectance of the plated article may be reduced by rendering the exterior article matte in appearance prior to the final plating operation. Thus, different effects can be achieved by varying the surface condition of the base material or underplate.

It will be appreciated to those skilled in the art that the process involves a chromic acid electrolyte having an extremely low concentration of sulfuric acid compared to the concentration of sulfuric acid in conventional CrO baths, and with the addition of axalic acid, producing the heretofore unexpected result of obtaining a lustrous iridescent plating.

EXAMPLES Example 1.A conventional polyvinylchloride lined steel tank containing a lead alloy anode is used in this example and in the following examples. A zinc die cast automobile emblem was prepared for plating according to conventional procedures as heretofore described. The emblem was copper and nickel plated according to conventional plating procedures. After rinsing, the emblem was transferred to the steel tank Which contained the iridescent plating bath which comprised a solution containing 45 grams per liter of chromic anhydride and .21 gram per liter of sulfuric acid, and 14 grams per liter of oxalic acid. The current density was 200 amperes per square foot and the temperature of the bath was 112 F. The plating time was three minutes. The emblem was then removed and rinsed in cold water, then hot water, and dried. The emblem has a coating of chrome which had a greenish iridescent luster.

Example 2.A brass ash tray was prepared for plating as in the above examples and then copper and nickel plated according to conventional procedures. Thereafter, it was rinsed and transferred to an iridescent plating bath which contained 55 grams per liter of chromic anhydride and 0.26 gram per liter of sulfuric acid, and 18 grams per liter of oxalic acid. The current density was 250 amperes per square foot and the temperature of the bath was 107% F. The ash tray was plated for a period of five minutes and then removed from the bath and rinsed in cold water, then hot water, and dried. The ash tray had a chrome deposit which had a bluish iridescent luster.

Example 3.A brass drawer handle was prepared for plating as in the above examples and then copper and nickel plated according to conventional procedures. Thereafter, it was rinsed and transferred to an iridescent plating bath which contained 50 grams per liter of chromic anhydride and 0.23 gram per liter of sulfuric acid, and 16 grams per liter of oxalic acid. The current density was 220 amperes per square foot and the temperature of the bath was 118 F. The drawer handle was plated for a period of six minutes and then removed from the bath and rinsed in cold water, then hot water, and dried. The drawer handle had a chrome deposit which had a reddish iridescent luster.

What is claimed is:

1. A process of producing electro-depositcd iridescent chromium coatings comprising plating an object as cathode in an aqueous bath comprising about 15 to about 450 grams per liter of chromic anhydride and about 0.07 to about 2.1 grams per liter of sulfuric acid, and about 1 to 75 grams per liter of oxalic acid, said oxalic acid being supplied by an equivalent amount of a compound selected from the group consisting of oxalic acid and ferric ammonium oxalate, using a current density of about to about 600 amperes per square foot and a temperature of about F. to about 140 F., for a period of time sufi'icient to give a chromium coating of desired thickness.

2. The process of claim 1 wherein said bath comprises about 35 to about 55 grams per liter of chromic anhydride and about 0.16 to about 0.26 gram per liter of sulfuric acid, and about 10 to 20 grams per liter of oxalic acid.

3. The process of claim 1 wherein equivalent amounts of sodium sulfate are used in place of the sulfuric acid.

4. The process of claim 1 wherein equivalent amounts of ferric ammonium oxalate are used to supply said oxalic acid.

5. The process of claim 1 wherein said current density is about 150 to about 250 amperes per square foot;

6. The process of claim 1 wherein said temperature is about F. to about F.

7. The process of claim 1 wherein said period of time is about 30 seconds to about ten minutes to obtain decorative iridescent coatings.

8. The process of claim 1 wherein said object to be plated has a previously deposited layer of metal other than chromium.

9. The process of claim 1 wherein the object to be plated is the cathode and the anode is a lead alloy anode.

10. The process of claim 9 wherein the anode area is at least as large as the cathode area.

References Cited UNITED STATES PATENTS 3,311,548 3/1967 Brown et al. 2405l FOREIGN PATENTS 22,855 11/ 1892 Great Britain. 22,856 12/ 1892 Great Britain. 379,987 9/ 1932 Great Britain. 143,429 12/1953 Sweden.

HOWARD S. WILLIAMS, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A PROCESS OF PRODUCING ELECTRO-DEPOSITED IRRIDESCENT CHROMIUM COATINGS COMPRISING PLATING AN OBJECT AS CATHODE IN AN AQUEOUS BATH COMPRISING ABOUT 15 TO ABOUT 450 GRAMS PER LITER OF CHROMIC ANHYDRIDE AND ABOUT 0.07 TO ABOUT 2.1 GRAMS PER LITER OF SULFURIC ACID, AND ABOUT 1 TO 75 GRAMS PER LITER OF OXALIC ACID, SAID OXALIC ACID BEING SUPPLIED BY AN EQUIVALENT AMOUNT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF OXALIC ACID AND FERRIC AMMONIUM OXALATE, USING A CURRENT DENSITY OF ABOUT 50 TO ABOUT 600 AMPERES PER SQUARE FOOT AND A TEMPERATURE OF ABOUT 60*F. TO ABOUT 140*F., FOR A PERIOD OF TIME SUFFICIENT TO GIVE A CHROMIUM COATING OF DESIRED THICKNESS. 